The present invention relates generally to programmable attenuators, and more particularly to an electrically controllable attenuator particularly suited for high frequency applications.
Since digital controllers such as mini-computers and microprocessors are readily available in the market place, it is becoming more and more desirable to automate electrical functions in automatic test and measurement instruments, such as oscilloscopes, wherein many controls are often required to provide a selection of parameters for a desired test set up. One such test set up is the selection of attenuation of an input signal.
One prior art programmable attenuator is that taught by Murphy in U.S. Pat. No. 4,121,183, assigned to the assignee of the present invention. This is an RC attenuator employing field-effect transistor (FET) switching. There are several disadvantages associated with this type of programmable attenuator. Finite resistance of the switching FET when turned on causes attenuation error and thermal instability. A switchable amplifier is required therein in order to obtain more than two subdecimal attenuation factors; however, this switchable gain amplifier results in a difficulty in maintaining a constant frequency response.
It is therefore an object of the present invention to provide an improved programmable attenuator overcoming the foregoing disadvantages of the prior art attenuator.
It is another object of the present invention to provide a switchable attenuator suitable for high-frequency applications.
It is yet another object of this invention to provide a programmable attenuator particularly suited for the vertical input circuit of an oscilloscope.
Other objects, features, and advantages of the present invention will become apparent to those having skill in the art upon a reading of the following description when taken in conjunction with the accompanying drawings.